Fuel cells are envisaged as a system of electrical power supply for future mass-produced motor vehicles, as well as for a great number of applications. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. Hydrogen (H2) or molecular hydrogen is used as a motor fuel for the fuel cell. Hydrogen (H2) or molecular hydrogen is oxidized and ionized on an electrode of the cell and oxygen (O2) or molecular oxygen in the air is reduced on another electrode of the cell. The chemical reaction produces water at the cathode, oxygen being reduced and reacting with the protons. The great advantage of the fuel cell is that it averts the release of atmosphere-polluting compounds at the place where the electricity is generated.
Proton-exchange membrane (PEM) fuel cells have particularly interesting properties of compactness. Each cell unit of the fuel cell comprises an electrolytic membrane enabling only the passage of protons and not the passage of electrons. The membrane comprises an anode on a first face and a cathode on a second face to form a membrane/electrode assembly known as an MEA.
At the anode, molecular hydrogen is ionized to produce protons passing through the membrane. The electrons produced by this reaction migrate to a flow plate and then pass through an electrical circuit external to the cell unit to form an electrical current.
The fuel cell can comprise several flow plates, for example made of metal, stacked on one another. The membrane is positioned between two flow plates. The flow plates can include channels and holes to guide the reactants and the products to and from the membrane. The plates are also electrically conductive to form collectors of the electrons generated at the anode. Gas diffusion layers are interposed between the electrodes and the flow plates and are in contact with the flow plates.
The most widespread method for producing hydrogen from water consists in using the principle of electrolysis. To apply such methods, electrolyzers provided with a proton-exchange membrane are known. In such an electrolyzer, an anode and a cathode are fixed on either side of the proton-exchange membrane (to form the membrane/electrode assembly) and are put into contact with water. A difference in potential is applied between the anode and the cathode. Thus, oxygen is produced at the anode by oxidation of the water. The oxidation at the anode also generates H+ ions that pass through the proton-exchange membrane up to the cathode and electrons are sent back to the cathode by the electrical power supply. At the cathode, the H+ ions are reduced to generate molecular hydrogen.
In practice, such an electrolyzer generally comprises power supply plates positioned on either side of the membrane/electrode assembly. Current collectors are disposed between the power supply plates and the membrane/electrode assembly.
Such an electrolysis device has undesirable effects. One of the challenges entailed by such a proton-exchange membrane electrolyzer is that of augmenting its efficiency, increasing its service life, reducing its manufacturing cost and ensuring a high level of security. The challenges are identical for a proton-exchange membrane fuel cell. These parameters are highly dependent on the method for manufacturing electrodes.
According to a first type of method for manufacturing a membrane/electrode assembly, catalyst ink is deposited in layers on the gas diffusion layers. After this ink has been dried, the membrane/electrode assembly is made by hot pressing. Hot pressing optimizes the contact between an electrode and the membrane in order to reduce their contact resistance to the maximum and thus optimize the performance of the MEA.
The hot-pressing operation unfortunately causes a deterioration of the membrane and notably affects the service life of the MEA. Owing to the great roughness of the gas diffusion layers, considerable pressure has to be applied during the hot-pressing process. Such pressure also greatly affects the porosity of the gas diffusion layers and therefore the performance of the device including the MEA.
To resolve some of these drawbacks, a second type of manufacturing method proposes to directly deposit electrocatalyst ink on the proton-exchange membrane so as to form an electrode-forming layer.
Such a method causes deformation by wet expansion or hydro-expansion of the membrane during the deposition of the ink. The method then gives rise to a deformation of the membrane by retraction under drying. This deformation is non-negligible and causes mechanical stresses at the deposits which may lead to cracks in the electroactive layer formed. Such cracks reduce the electron percolation of the electrode and thus reduce its electrical conductivity. In addition, the cracks may alter the cohesion between the electrode and the membrane. Thus, a non-negligible part of the electrode can become non-functional.
Besides, during operation, the membrane is totally submerged in water, giving it a maximum expansion rate. The mechanical stresses at the interface between the electrode and the membrane are thus the maximum, causing increased deterioration of the electrode. This deterioration of the electrode reduces the energy efficiency of the electrolyzer as well as its service life.
Another problem of the formation of an electrode by catalytic deposition on the membrane is the damage to this membrane by the solvents (for example ethanol or isopropanol) present in ink. Secondly, the solvents cause an increase in the permeability of the membrane to gases. A part of the gases produced at the anode and at the cathode thus passes through the proton-exchange membrane by diffusion. This causes firstly problems of purity of the gases produced but also problems of security. The proportion of hydrogen in the oxygen must especially remain absolutely below 4%, such a proportion being the lower limit of explosiveness of hydrogen in oxygen. Besides, the damage to the membrane by the solvents reduces its service life.
A third method of manufacture of the membrane/electrode assembly gives an optimal compromise between the performance of the MEA and its service life. This method comprises a preliminary step for printing the electrocatalyst layer on a smooth and hydrophobic support, insensitive to the solvents present in the ink. The printing support especially has a very low surface energy and a very low roughness. After the formation of the electrode by drying of the catalyst ink, the electrode is assembled with the membrane by hot pressing. Because of the low adhesion of the electrode to the printing support, this hot pressing can be done at low temperature and pressure. The deterioration of the membrane during the hot-pressing step is thus reduced. In addition, the electrode formed by printing on a smooth support has a homogenous thickness and composition, thus also limiting deterioration of the membrane during the hot pressing. In addition, since the electrode is assembled on the membrane after drying, the membrane is not put into contact with the ink solvents and undergoes no corresponding deterioration.
The document US2008/0105354 describes such a method of membrane/electrode assembly for a fuel cell. The membrane/electrode assembly formed has reinforcements or subgaskets. Each reinforcement surrounds the electrodes. The reinforcements are made out of polymer films and reinforce the membrane/electrode assembly at the inlets of gas and of cooling liquid. The reinforcements facilitate the handling of the membrane/electrode assembly to prevent its deterioration. The reinforcements also limit the dimensional variations of the membrane according to temperature and humidity. In practice, the reinforcements are superimposed on the periphery of the electrodes in order to limit the phenomenon of permeation of gas which is the cause of a deterioration of the membrane/electrode assembly.
According to this method, a reinforcement is made by forming an aperture in the median part of a polymer film. The reinforcement comprises a pressure-sensitive adhesive on one face. A membrane/electrode assembly is recovered and the aperture of the reinforcement is placed plumb with an electrode. The reinforcement covers the periphery of this electrode. A pressing operation is then carried out to fixedly attach the reinforcement to the membrane and to the border of the electrode by means of adhesive. As a variant, a reinforcement without adhesive can be fixedly attached to the membrane during the hot-pressing step, using a pressing temperature greater than the glass transition temperature of the membrane. The gas and liquid inlets are then cut out of the reinforcement.
Owing to the excess thickness between the electrode and the reinforcement, the pressure and the temperature must be increased when the assembling is done, to the detriment of the service life of the membrane. Besides, the method of manufacture remains relatively complex and increases the number of steps that could cause a malfunctioning in the membrane/electrode assembly.
The document JP2010 129435 A describes a method for manufacturing a membrane/electrode assembly with steps of hot adhesion between a proton-exchange membrane and a mask in the shape of a frame comprising two layers of deposit of an electrocatalyst ink on the membrane through the aperture of the mask and of withdrawal of a layer forming the mask.
This method leads to deterioration of the membrane and/or of the electrodes during its execution.